How to copy an action strip from one object to another?
Assuming, both armatures have same bone names (etc) 
just did a quick test, NLA editor won't let you copy action strips from one object to another via dragging / moving (G key) .

Option 1 (shift + A) in NLA editor will let you pick action as well


Option 2 (as shown in image)
Right now, best way to do is to use 'Action Editor' in Dope Sheet Area. Just use the action there on armature and then use it in NLA